Process and apparatus for separating a carbon dioxide-rich gas by distillation

ABSTRACT

In a process for separating a gas rich in carbon dioxide by distillation, the gas rich in carbon dioxide is cooled in a heat exchanger and is sent to a distillation column, a gas depleted in carbon dioxide is withdrawn from the top of the column and a liquid enriched in carbon dioxide is withdrawn from the bottom of the column, a gas enriched in carbon dioxide is compressed in a compressor down to a first pressure and at least one portion of the gas enriched in carbon dioxide originating from the compressor is expanded from the first pressure to a second pressure lower than the first pressure, for example the pressure of the column, and the expanded gas is used to heat the bottom of the column.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International Application PCT/FR2012/052417, filed Oct. 23, 2012, which claims priority to French Application No. 1160039, filed Nov. 4, 2011, the entire contents of which are incorporated herein by reference.

BACKGROUND

This invention relates to a process and apparatus for separating a carbon dioxide-rich gas by distillation.

It is known that a mixture containing carbon dioxide as one of its main components can be separated by distillation. A carbon dioxide-rich gas can contain at least 50% of carbon dioxide, or even at least 70% of carbon dioxide, and sometimes at least 90% of carbon dioxide. All percentages in this document related to purities are percentages by volumes on a dry basis. The remainder of the gas can include at least one gas among nitrogen, oxygen, argon, hydrogen, carbon monoxide and methane. The mixture may originate from an oxy-combustion process wherein a mixture of oxygen and fuel burns in a boiler to produce a residual gas, at least partly composed of the gas to be separated according to the invention, produced after purification.

A CO₂ distillation column is necessary if high purity CO₂ (>90% v, preferably >95% v and more specifically >99% v) production is required. The liquid (or two-phase) CO₂ partially purified by drying and partial condensation is distilled against pure gaseous CO₂. This pure CO₂ introduced into the column sump to purify the liquid can originate from the purified CO₂ compressor of the unit. In this case, CO₂ is necessary at a higher pressure or at the same pressure as the column (the column pressure being between 6 and 70 bars, and more particularly between 10 and 16 bars). All pressures given in this document are absolute pressures.

Most processes of this type based on distillation to purify the mixture use a distillation column, the sump of which is heated by the mixture to be distilled.

EP-A-2381198 describes a process according to the preamble to claim 1.

In the case of U.S. Pat. No. 3,498,067, DE-A-3639779 and JP-A-56077673, the mixture is sent to the column and according to documents FR-A-2934170, EP-A-1953486, US20020059807, U.S. Pat. No. 3,130,026, WO-A-2006054008, WO-A-2007126972, EP-A-0965564, EP-A-0994318, the liquid in the sump vaporises against the mixture in a dedicated exchanger, separated from another larger exchanger that cools the mixture.

EP 1953486 and EP 2002190 disclose the concept of reboiling of the distillation column by the use of part of the heat introduced by the feed gas.

FR-A-2934170 discloses the use of the carbon dioxide enriched gas produced, to reboil the column.

SUMMARY

This invention consists of drawing off the CO₂ necessary for reboiling the column from CO₂ production on the downstream side of the cold box. Steam injected into the distillation column sump directly will then be derived from the final compression or the CO₂ cycle or any other purified CO₂ compressor. Different methods can then be envisaged. CO₂ gas can be drawn off before it reaches its final pressure, in other words between compressor stages, for example in the case of a multi-stage compressor. This CO₂ can be sent directly into the column.

The higher the pressure to which CO₂ is compressed, the more the expansion cools the gas available to reboil the column. Therefore, the CO₂ pressure at the outlet from the compressor can be chosen so that CO₂ at the highest pressure is available, so that a greater pressure reduction can be obtained. The CO₂ pressure reduction can add frigories, particularly in a turbine.

One purpose of the invention is a process for separation of a feed gas containing at least 50% of carbon dioxide by distillation wherein:

-   i) the rich feed gas is cooled in a heat exchanger and is     transferred into a distillation column, -   ii) carbon dioxide depleted gas is drawn off from the top of the     column and a carbon dioxide enriched liquid is drawn off from the     column sump, -   iii) a gas richer in carbon dioxide than the feed gas is compressed     to a first pressure in a compressor, characterised in that: -   iv) the pressure of part of the carbon dioxide enriched gas from the     compressor is reduced from a first pressure to a second pressure     lower than the first pressure, for example the pressure in the     column, and the expanded gas is used to heat the column sump.

According to other optional aspects:

-   at least part of the carbon dioxide enriched gas is produced by     vaporising at least part of the carbon dioxide enriched liquid drawn     off from the column. -   the carbon dioxide enriched gas originates at least partly from an     external source. -   at least part of the expanded gas is transferred into the column. -   at least part of the expanded gas is transferred into the column to     be separated. -   at least part of the expanded gas is transferred into a vaporiser in     the column sump to heat part of the liquid in the column sump. -   a second part of the carbon dioxide enriched gas is compressed in     the compressor to a third pressure higher than the first pressure. -   at least part of the gas at the third pressure forms an end product     of the process. -   at least part of the gas from the compressor forms a refrigeration     cycle gas for the process and is then condensed, cooled, expanded,     heated and returned to the compressor. -   the second pressure is higher than the pressure in the column by at     least 1 bar or is lower than the pressure in the column. -   the third pressure is between 50 and 200 bars.

Another purpose of the invention relates to apparatus for separation of a feed gas containing at least 50% of carbon dioxide by distillation comprising a heat exchanger, a distillation column, means of transferring the feed gas into the heat exchanger to be at least partially liquefied, means of transferring the at least partially liquefied fluid from the heat exchanger into the column, means of drawing off a carbon dioxide depleted gas from the top of the column, means of drawing off a carbon dioxide enriched liquid from the column sump, a compressor, means of transferring a gas richer in carbon dioxide than the feed gas to the compressor and a pressure reduction means, characterised in that it comprises means of transferring the carbon dioxide enriched gas at a first pressure from the compressor to the pressure reduction means to reduce its pressure to a second pressure, and means of transferring the carbon dioxide enriched gas to the pressure reduction means to the column or a vaporiser in order to vaporise a liquid in the column sump.

The pressure reduction means may be a valve or a turbine.

It is also possible to use a dedicated exchanger that will act as a reboiler. CO₂ originating from the column sump is vaporised against CO₂ originating from the final compression or from the CO₂ cycle or any other CO₂ compressor. This CO₂ is then cooled and even condensed as a function of its pressure and its temperature. The advantage of having a dedicated exchanger is that the CO₂ from the process is not mixed with the CO₂ from a chiller, for example in the case of a liquefier.

The CO₂ thus cooled can be used cold as a gas or it can be directly liquefied in the reboiler for subsequent use in the same process or in another process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one preferred embodiment of the invention.

FIG. 2 illustrates another preferred embodiment of the invention.

FIG. 3 illustrates another preferred embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will be described in more detail with reference to the figures that show apparatus according to the invention.

FIG. 1 shows oxy-combustion apparatus 1 (or another source) that produces fumes purified to produce a carbon dioxide-rich gas containing at least 50%, or even at least 70% of carbon dioxide. The gas 2 contains at least one other impurity, for example nitrogen, oxygen, argon, hydrogen, carbon monoxide or methane. The gas 2 is at least partially liquefied in a heat exchanger 3 to produce a fluid 4 that is transferred to the top of a distillation column 5. It is separated there to form a head gas 6 enriched with at least one compound among nitrogen, oxygen, argon, hydrogen, carbon monoxide and methane. The gas 6 is depleted in carbon dioxide. The sump liquid 7 is depleted in at least one compound among nitrogen, oxygen, argon, hydrogen, carbon monoxide and methane. The carbon dioxide enriched liquid 7 contains at least 60% of carbon dioxide, preferably at least 70% of carbon dioxide and is sent to a storage 9. The liquid 11 drawn off from storage can be an end product.

According to one variant of the invention, as shown in dashed lines, the liquid 7 is transferred through the conduit 13 to the vaporiser 15 to form a gas. This gas is compressed in a compressor 17 composed of one, or possibly four or even six to eight stages, 17A, 17B, 17C, 17D, 17E, 17F. The gas compressed in all stages is a gas product 19 at a third pressure between 50 bars and 200 bars depending on the application.

The pressure of the gas 21 compressed in the first three stages 17A, 17B, 17C is reduced from a first pressure to a second pressure in a valve 23 and is then cooled in the exchanger 3 and injected directly into the sump of the column 5. The valve 23 can also be downstream from the exchanger 3.

Alternatively, the gas 21 may be cooled in an exchanger that is not the exchanger 3.

The gas can even be transferred into the column at the outlet temperature from the compressor stage.

The column 5 functions at between 6 and 70 bars, and more particularly between 10 and 16 bars.

FIG. 2 shows that the gas 21 can be the gas 21A diverted downstream from stage 17A, the gas 21B downstream from stage 17B, the gas 21C diverted downstream from stage 17C, diverted downstream from stage 17D, the gas 21E diverted downstream from stage 17 E or the gas diverted downstream from stage 17F. Similarly, valve 23 may be replaced by a turbine 25 upstream or downstream from cooling before entering into the column. This makes it possible to benefit from the pressure difference by chilling.

FIG. 3 is different from FIG. 2 in that the gas at reduced pressure after valve 23 as far as the second column is not transferred into the column but rather to a vaporiser 31 outside the column. The vaporiser 31 is also supplied by sump liquid 26 from the column 5. The vaporised liquid is returned to the column. The gas 21 transferred to the vaporiser is liquefied to form the liquid 29 for subsequent use in the process or in another process.

The compressor 17, or some stages of the compressor, can form a cycle compressor in the unit.

The compressed gas in the compressor 17 may be a fully or partly vaporised liquid 11. Otherwise some or all of the gas can originate from an external source, for example a pipe or other equipment.

For all examples, the pressure in the column 5 can vary between 6 bars and 70 bars and may contain structured linings or trays. The gas 21 that is used for reboiling may be at the pressure in the column, in the case of direct injection as shown in FIGS. 1 and 2. Otherwise, the gas sent to the external reboiler 31 may be at a higher pressure than the pressure in the column (external reboiling and condensation) or at a lower pressure (external reboiling on sensible heat alone). Thus, the second pressure can be lower or higher than the pressure in the column, or identical to the pressure in the column. If the second pressure is higher than the pressure in the column, it is at least 1 bar higher than the pressure in the column. The pressure in the column is conventionally the pressure measured at the middle of the column.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1. A process for separating a feed gas comprising at least 50% of carbon dioxide by distillation, comprising: i) Cooling the feed gas in a heat exchanger and transferring the cooled feed into a distillation column, ii) withdrawing carbon dioxide depleted gas from the top of the column and withdrawing a carbon dioxide enriched liquid relative to the carbon dioxide-rich gas from the column sump, iii) compressing a gas richer in carbon dioxide than the feed gas is in a compressor to a first pressure wherein the pressure of at least a part of the carbon dioxide enriched gas from the compressor is reduced from a first pressure to a second pressure lower than the first pressure, and the expanded gas is used to heat the column sump.
 2. The process according to claim 1, wherein at least part of the carbon dioxide enriched gas is produced by vaporising at least part of the carbon dioxide enriched liquid drawn off from the column.
 3. The process according to claim 1, wherein the carbon enriched dioxide gas originates at least partly from an external source.
 4. The process according to claim 1, wherein at least part of the expanded gas is transferred into the column.
 5. The process according to claim 1, wherein at least part of the expanded gas is transferred into a vaporiser in the column sump to heat part of the liquid in the column sump.
 6. The process according to claim 1, wherein a second part of the carbon dioxide enriched gas is compressed in the compressor to a third pressure higher than the first pressure.
 7. The process according to claim 6, wherein at least part of the gas at the third pressure forms an end product of the process.
 8. The process according to claim 1, wherein at least part of the gas from the compressor forms a refrigeration cycle gas for the process and is then condensed, cooled, expanded, heated and returned to the compressor.
 9. The process according to claim 1, wherein the second pressure is higher than the pressure in the column by at least 1 bar or is lower than the pressure in the column.
 10. An apparatus for separating a feed gas containing at least 50% of carbon dioxide by distillation comprising a heat exchanger, a distillation column, means of transferring the feed gas into the heat exchanger to be at least partially liquefied, means of transferring the at least partially liquefied fluid from the heat exchanger into the column, means of drawing off a carbon dioxide depleted gas from the top of the column, means of drawing off a carbon dioxide enriched liquid from the column sump, a compressor, means of transferring a gas richer in carbon dioxide than the feed gas to the compressor and a pressure reduction means, characterized in that it comprises the means of transferring the carbon dioxide enriched gas at a first pressure from the compressor to the pressure reduction means to reduce its pressure to a second pressure and means of transferring the carbon dioxide enriched gas from the pressure reduction means to the column or a vaporiser in order to vaporise a liquid in the column sump.
 11. The apparatus according to claim 10 wherein the pressure reduction means is a turbine.
 12. The apparatus according to claim 10, comprising a means of sending the carbon dioxide enriched gas from the pressure reduction means to the column. 